This invention relates to a solid-state imaging device, and more particularly, to a solid-state imaging device used in a high sensitivity color camera.
In the case of constituting a color camera by using a solid-state imaging device or devices, the structure of the three imaging device type, the single imaging device or two imaging device type is employed. The configuration of a color camera of the three imaging device type is first shown in FIG. 1. A white light W incident to an imaging lens 1 and passed therethrough is separated into blue light B, red light R and green light G by a dichroic prism 2. These rays of light are incident to three solid-state imaging devices 3, 4 and 5 provided in respective light paths. Thus, a color image signal is provided.
In actual terms, only the blue light B of the white light W passed through the imaging lens 1 is reflected by a dichroic surface 6. After being reflected on a reflection mirror 7, the blue light B is incident to the solid-stage imaging device 3 for blue. The red light R of the rays of light which not reflected by the dichroic surface 6 is reflected by a dichroic surface 8. After reflected by a being reflection mirror 9, the red light R is incident to the solid-state imaging device 4 for red. The remaining green light G is not reflected by any one of dichroic surfaces, and is incident to the solid-state imaging device 5 for green. Then, photoelectric conversion is carried out at the solid-state imaging devices 3, 4 and 5. Thus, a color image signal is provided from three electric signals outputted therefrom.
This three imaging device type camera has a high sensitivity because the white light W is optically separated to use rays of light of all wavelength bands. Furthermore, it is possible to set the number of pixels corresponding to pixels of a monochromatic image for every color, resulting in excellent resolution and good picture quality. However, since three solid-stage imaging devices 3, 4 and 5 are used for one camera, not only the number of parts is increased, but also a dichroic prism which is in a complicated form and is expensive because of necessity of reflection surfaces corresponding to wavelength bands of respective colors is required. Furthermore, the precise positioning between the solid-state imaging devices 3, 4 and 5 and the optical system including the imaging lens 1 and the dichroic prism 2 is required. As a result the camera becomes large and expensive because of the space required for positioning. For this reason, cameras of this system only have a limited use such as use for broadcasting, or business use, etc.
The configuration of a color camera 10 of the single imaging device is shown in FIG. 2. A white light W passed through an imaging lens 11 is passed through a color filter 12, and is then incident to a single monochromatic solid-state imaging-device 13. The color filter 12 is such that respective portions of red R, green G and blue B are, arranged in a mosaic form as shown in FIG. 3. For one color filter 12 of this kind, there is a color in which a filter element printed on a glass base plate is tightly fixed on the chip for a monochromatic device, and there is a color filter in which such a filter element is formed on a chip for a monochromatic device directly by patterning.
A signal outputted from the solid-state imaging device 13 is inputted to a color separation circuit 15, at which it is separated into signals G, R and B of the three primary colors. The green signal G of these signals is provided by interchangeably sampling a signal delayed by an 1H delay circuit 14 and an original signal per each pixel and adding them. Respective color signals go through low-pass filters 16a, 16b and 16c, process amplifiers 17a, 17b and 17c, and a high-frequency component separation circuit 18. Thus, a color image signal V corresponding to three color light is synthesized by a color encoder 19.
Meanwhile, there are many instances where the color filter is contained in the package such that it is it is included in a semiconductor chip. For this reason, the optical system can be constructed in the same manner as in the monochromatic imaging device Thus, the camera can be miniaturized and reduced in cost. However, rays of light except for respective specific wavelengths must be absorbed for every color component by such a color filter 12. As a result, only one third of the entire quantity of an incident light reaches the pixel unit of the solid-state imaging device 13. For this reason, lowering of the sensitivity cannot be avoided when compared with the camera of the three solid-stage imaging device. In addition, the number of pixels corresponding to respective wavelength bands is reduced to one third of that of the monochromatic imaging device, and the spatial resolution is also lowered.
The camera of the two solid-state imaging device type is constructed to synthesize signals from two solid-state imaging devices to provide a signal corresponding to three wavelength bands. This camera is adapted to compensate for the drawbacks of the camera of the three solid-state imaging device type as well as the camera of the single solid-state imaging device type. However, the camera of the two solid-state imaging device type has an intermediate property in the dimension, cost, sensitivity, and resolution. Therefore, the camera of this type cannot essentially solve the problems with the camera of the three solid-state imaging device type and the camera of the single solid-state imaging device type.
As described above, even in the case where any system of color cameras using conventional solid-state imaging device is selected, the camera employed could not satisfy the requirements of dimension and sensitivity, etc.